Rectal transmission of HIV is the most common route of infection during homosexual intercourse in developed countries, and anal sex increases the risk of male to female transmission in developing countries. The mechanism of protective immunity induced by rectal vaccination differs from that induced by systemic immunization. The objectives of this proposal are to develop an immunization strategy in non-human primates that will elicit 3 levels of immunity which match 3 phases of HIV infection (a) To induce rectal secretory IgA antibodies and cytotoxic T cells to prevent rectal infection by SIV. (b) To elicit regional lymph node proliferative and cytotoxic T cells and specific B cells to prevent the development of a reservoir of latency of the virus in these lymph nodes. (c) To induce specific serum neutralizing antibodies and circulating cytotoxic T cells, in order to prevent dissemination of the virus. The project has been designed to overcome a low degree of rectal immunity. This might be achieved (i) by increasing the binding affinity of the vaccine to two mucosal receptors; GM1 ganglioside and Fcgamma receptors. Recombinant envelope gp160, core p27 and reverse transcriptase will be used to link to cholera toxin subunit B (CT-B) and to IgG class of antibodies. (ii) The rectal associated lymphoid tissue will be augmented by gut, nasal or bronchial associated lymphoid tissue, through oral or nasal administration of the vaccine. The immunological changes involving antibodies and T cell functions will be assayed sequentially. IgA and IgG antibodies will be assayed by ELISA, using rectal washings, urine, serum and saliva. To overcome the unpredictable dilution factor inherent in collecting rectal washings, the IgA antibodies will be quantitated in terms of affinity purified antibody, and expressed as a percentage of total IgA concentration in rectal washings. T cell proliferative and cytotoxicity assays will be carried out with PBMC before and after each immunization, and at autopsy with iliac, mesenteric, and other lymph nodes, and spleen. T cell epitopes will be mapped by stimulating short term cell lines with overlapping peptides of the sequenced antigens and the resulting peptides will be applied to T cell cytotoxicity assays. Antibody forming B cells and cytotoxic T cells will be assayed directly by separating lymphocytes from the rectal tissue. The triple level of immunity to SIV in the rectal mucosa, regional lymph nodes and blood will then be tested by rectal challenge of macaques with live SIV. Pre-titrated stocks of well characterized SIVmac 32H and molecular clones will be used to challenge vaccinates by the rectal route. The macaques will be monitored for infection by virus isolation, PCR and seroconversion. Alternative vaccines will be tried in the SIV model, using cholera and salmonella vectors, in order to develop the most effective rectal immunity of SIV infection. These experiments should pave the way in the development of an effective strategy of rectal immunization, in preventing human transmission of HIV infection by the anal route.